Hey there! Are you washing your hair with just water? If so, then it may be that your hair is responding well to No Poo, but your scalp is the kind that produces a bit more oil than the average scalp, and thus needs to be cleaned with something additional to water once in a while.
The nopoomethod.com website includes various suggestions for things you can use to clean your hair, aside from commercial shampoo. The most natural and readily available alternatives that come to mind are baking soda (BS) and apple cider vinegar (ACV).
According to the site, BS "is effective for washing away oil buildup on your scalp and hair." It's a great solution for oilier scalps. However, the problem is that it might leave your hair a bit dry. So this is where ACV comes in. ACV is not only an effective germ killer and fungicide; it's also a natural moisturizer, so it compensates for the BS's unwanted drying effect.
Why don't you try doing this twice a week: Wash with BS, rinse with water, then wash again with ACV, and rinse again with water. I hear there are people who shampoo with BS, and then rinse with ACV, and are seeing nice results; you might try that too. This should help your scalp feel and look cleaner within a couple weeks.
Another possibility might be to purchase one of those natural/organic shampoos that are very non-aggressive with your scalp and hair, though they are also more expensive.
Hope this helps. Best of luck! Let me know how it goes.
You may want to read through these pages, as they provide tips on how to apply the BS and ACV: